1. Field of the Invention
The present invention relates to a unique word detection circuit suitably employed when establishing frame synchronization by applying time division multiple access to the reverse line from a base station to a mobile station. In particular, the present invention relates to a unique word detection circuit that employs a PN (pseudo noise) pattern repeat preamble signal.
This application is based on.Japanese Patent Application No. Hei 10-200854, the contents of which are incorporated herein by reference.
2. Background Art
In data transmission such as performed in a 2.4 GHz band wireless LAN (local area network) system, it has been the conventional practice to employ a means for detecting a unique word included in the received signal when establishing frame synchronization. The frequency bandwidth which can be employed in this type of system is restricted, and a spectrum spread system is employed because the frequency band is shared in common with that used by ISM (industrial scientific and medical equipment) band microwave ranges, medical equipment and the like. An explanation will be made using as an example IEEE 802.11 (The Institute of Electrical and Electronic Engineers, Inc. 802.11), the international standard method for this wireless LAN.
FIG. 6 is a block diagram for explaining an example of the structure of a IEEE 802.11 unique word detection circuit according to the conventional technology. The wireless LAN receiving device shown in FIG. 6 is composed of an antenna 8, radio 9, inverse spread device 10, and UW detector (unique word detector) 11. The receiving device in FIG. 6 receives data transmitted from a base station via antenna 8. The data received from antenna 8 undergoes frequency conversion at radio 9, and is output to inverse spread device 10. Inverse spread device 10 determines the self-interaction correlation value, and outputs a received data signal (F) to UW detector 11. UW detector 11 compares sequentially output received data signals (F) with a preset 16 symbol unique word pattern. If there is a match between a received data signal (F) and the preset unique word pattern, UW detector 11 outputs a 1 pulse UW detection signal (G). Frame synchronization is established based on this one pulse signal at the receiving device, and data uptake begins.
The preamble portion of the received signal in IEEE 802.11 is designed such that there is a 192 symbol signal attached to the head for the purpose of establishing synchronization at each burst. A 16 symbol long unique word pattern is also present at a predetermined position in the preamble signal array.
FIG. 7 is a timing diagram showing an example of the operation for detecting the unique word under normal reception at the receiving device shown in FIG. 6 in IEEE 802.11 according to the conventional technology. UW detector 11 compares the 16 symbol unique word that is included in the 192 symbol preamble signal array present in the received data signals (F) that are being sequentially output from inverse spread device 10, with a preset 16 symbol unique word pattern. When a match occurs, UW detector 11 outputs a 1 pulse UW detection signal (G).
FIG. 8 is a timing diagram showing an example of the operation for detecting a unique word during abnormal reception in IEEE 802.11 according to the conventional technology. When an error occurs in the 16 symbol unique word in the 192 symbol preamble signal array that is attached to the head in received data signals (F) that are being sequentially output from inverse spread device 10 shown in FIG. 6, the results of the comparison with the 16 symbol unique word pattern will indicate non-matching. Accordingly, the 1 pulse detection signal is not output. UW detector 11 also performs a comparison on the information data. If the same pattern as the UW signal is present in the information data, a one pulse UW detection signal (G) is output. In other words, although the probability that the same unique word pattern signal will be present in the information data is small, if such a pattern is present, then an incorrect detection results.
Conventionally known technologies related to the present invention include a device such as disclosed in Japanese Patent Application, First Publication No. Hei 09-23221 for preventing incorrect detection of noise other than the unique word; a device such as disclosed in Japanese Patent Application, First Publication No. Hei 07-15423 for reducing the possibility that an aperture interval will be set at an incorrect position and reducing the rate of incorrect unique word detection; and a device such as disclosed in Japanese Patent Application, First Publication No. Sho 56-8916 for controlling the generation of the next aperture signal by setting the initial detection pulse to be the detection pulse for the true unique word, by means of stopping the aperture signal at the initial detection pulse.
In a conventional unique word detection method, it was necessary to lengthen the unique word in order to construct a system resistant to poor environmental conditions caused by noise, reflected waves and the like. However, transmission efficiency declines when the unique word is lengthened. Moreover, when an error occurs in the unique word signal in the preamble signal, a problem results in that a given pattern present in the information data that is the same as the unique word signal is detected in error.
The present invention was conceived in consideration of the above-described problems, and has as its objective the provision of a unique word detection circuit that can detect a unique word with excellent accuracy using a short unique word pattern, by employing a preamble signal that is a specified symbol repeat PN code, such as a 31 symbol repeat for example, and which can improve transmission efficiency and establish frame synchronization in a high-speed wireless communications system. It is a further objective of the present invention to provide a unique word detection circuit which can prevent incorrect detection of a unique word signal in the case where an error has occurred in the received data.
The present invention provides a correlation calculating processor for detecting the start of demodulated data in the received signal, and a unique word/gate signal generator for generating and outputting a gate signal for designating the time interval during which the unique word (UW) is detected based on the signal output by the correlation calculating processor. The present invention also provides a gate circuit for passing the received signal in a time interval based on the gate signal output from the unique word/gate signal generator, and an inverting circuit for inverting the signal output from the gate circuit. A pseudo noise (PN) detector compares the signal output from the inverting circuit with a predetermined unique word signal set value during the time interval in which the gate signal is being output from the unique word/gate signal generator, detects whether or not there is a match, and then outputs the results of the detection. A unique word detector compares the signal output from the gate circuit with a predetermined unique word signal set value during the time interval in which the gate signal is being output from the unique word/gate signal generator, and outputs the results of the comparison as the unique word detection signal. An error detector outputs a signal for halting unique word detection based on the signal output from the pseudo noise detector and the signal output from the unique word detector.
Further, the invention is characterized in that the received signal has a plurality of repeating pseudo noise codes consisting of a specific symbol number in the preamble signal of each burst, and in that a symbol indicating the predetermined unique word signal set value and a symbol indicating the value obtained by inverting the unique word signal set value are included respectively in at least two of the pseudo noise codes. Further, the invention is characterized in that the received signal has a plurality of repeating pseudo noise codes consisting of a specific symbol number in the preamble signal of each burst, and in that a symbol indicating the predetermined unique word signal set value is included in at least one pseudo noise code and a symbol indicating the value obtained by inverting the unique word signal set value is included in at least two or more pseudo noise codes.
Further, the invention is characterized in that the error detector outputs a signal for halting the unique word detection when the signal output from the pseudo noise detector indicates non-matching for the result of the comparison and the signal output from the unique word detector indicates non-matching for the result of the comparison. The invention is characterized in that the error detector outputs a signal for halting unique word detection when the signal output from the pseudo noise detector does not indicate a match for the result of the comparison, or when the signal output from the unique word detector does not indicate a match for the result of the comparison, during the time interval in which a gate signal is being output from the unique word/gate signal generator.
The present invention""s unique word detection circuit enables detection using a short unique word signal by employing a preamble signal which is a 31 symbol repeat pseudo noise code. In the present invention""s unique word detection circuit, the unique word can be detected by comparing a given UW pattern and PN pattern in the preamble signal of a 31 symbol repeat PN code, for example. As a result, the present invention resolves the problem of mistaken detection of a pattern in the information data that is the same as the unique word signal in the case where an error has occurred in the unique word signal in the preamble signal generated by a unique word detection circuit according to the conventional technology. The present invention further resolves the problem of reduced transmission efficiency due to the use of a long unique word.
The present invention restricts the time interval during which detection of a unique word is performed. As a result, it is possible to suspend detection of the burst signal when an error has occurred in the received data, so that incorrect detection of the same unique word pattern in the information data does not occur. In addition, by employing a preamble signal which is, for example, a 31 symbol repeat PN code, the present invention offers the effects of performing detection using a short unique word, reducing the transmission speed and decreasing the time required to establish synchronization. Moreover, in the present invention, it is possible to further prevent incorrect detection by mixing the unique word pattern and a pattern which is the inverse thereof in the preamble signal, or by making the pattern repeat number plural.